1. Field of the Invention
The present invention relates generally to an apparatus and method for determining a data transmission rate in a multi-hop relay system, and more particularly, to an apparatus and method for determining a data transmission rate in consideration of the queue length of a relay station in a multi-hop relay system.
2. Description of the Related Art
The development of a new fourth-generation (4G) mobile communication system is taking place to expand service coverage and provide a higher data transmission rate than in the third-generation (3G) mobile communication system. Many institutes and enterprises in advanced countries are already promoting competitive technology development for the upcoming 4G standardization.
The 4G mobile communication system operating in a high frequency band has a restricted data transmission rate and service coverage due to a high path loss. Recently, a multi-hop relay scheme has been researched to solve the above problem of the 4G mobile communication system. The multi-hop relay scheme uses one or more Relay Stations (RSs) to relay data and transmit a signal from a Base Station (BS) even to a Mobile Station (MS) remote from the BS, thereby making it possible to reduce a path loss, provide high-speed data transmission and expand service coverage.
In a single-hop system, because radio data transmission is performed only between a BS and an MS, the BS uses the Channel Quality Information (CQI) of the MS to determine a Modulation and Coding Scheme (MCS) level and thus a data transmission rate. However, in a multi-hop system, because a BS must manage not only data transmission between the BS and an MS but also data transmission between the BS and an RS, the BS must control a data transmission rate between the BS and the RS in consideration of all of the above data transmissions.
In a conventional single-hop system, because radio data transmission is performed only between a BS and an MS, communication can be performed within one frame when the BS transmits/receives data to/from the MS. Therefore, using the CQI report value of the MS for the previous frame, the BS allocates radio resources for the next frame and determines an MCS level. However, in a conventional multi-hop system, because several radio links exist, resource allocation and data rate determination must be performed on each of the radio links.
FIG. 1 illustrates the structure of a conventional multi-hop relay system.
Referring to FIG. 1, a Mobile Station MS1, which is located inside the service coverage of a BS, is connected through a direct link to the BS, while an MS2, which is located outside the service coverage of the BS and thus is incapable of communicating directly with the BS, is connected through an RS to the BS. That is, the RS is located between the BS and the MS2 to relay data from the BS to the MS2. A frame communicated between the BS and the RS will be referred to as a “frame A”, while a frame communicated between the RS and the MS2 (or MS1 in other Figures herein, when stated) will be referred to as a “frame B”.
A description will now be given of a process for allocating radio resources in such a 2-hop relay system on the basis of CQI information fed back from an MS. In a multi-hop system, because a BS transmits control information and data (traffic) to an RS and the RS relays the same to an MS, a BS-RS communication link between the BS and the RS and an RS-MS communication link between the RS and the MS must be distinguished from each other. For example, the BS-RS communication link and the RS-MS communication link must be distinguished from each other by dividing one frame into subframes or by defining two different frames.
The following description will be given assuming that the BS-RS communication link and the RS-MS communication link are distinguished from each other using different frames, as illustrated in FIG. 1.
A BS-RS radio link is provided with a Line Of Sight (LOS) connection and thus can provide more stable and rapid radio communication than a BS-MS radio link and an RS-MS radio link. Therefore, for BS-RS data communication, congestion does not occur and thus resource allocation and data rate determination can be performed in consideration of only RS-MS channel conditions. However, in the 2-hop relay system, a 2-frame (i.e., frame B and frame A) delay basically occurs because two hops are performed to transmit the CQI information of the MS to the BS, and a 1-frame (i.e., frame A) delay additionally occurs while the BS transmits data to the RS according to schedule based on the received CQI information.
Therefore, an MCS level and allocated resources used for transmission from the RS to the MS are determined based on the 3-frame previous CQI information. When the current channel condition is different from the 3-frame previous channel condition, the RS-MS data transmission efficiency degrades and a transmission failure frequently occurs. At this point, a feedback message ACKnowledgement/NonACKnowledgement (ACK/NACK) for informing a failure in the RS-MS transmission is transmitted to the BS through two hops. In this case, the BS has no choice but to transmit the next data to the RS without knowing the success/failure of the transmission of the previous data. That is, the BS continues to transmit data to the RS without detecting the success/failure of the transmission of the previous data, which causes data to be excessively loaded on the RS. When the RS buffers data excessively as described above, MSs serviced by the RS undergo an additional delay and jitter due to a change in the queue length of the RS.
There is an alternative method in which the BS simply forwards data, destined for the MS, to the RS without determining a data transmission rate based on the feedback information (CQI). However, the alternative method has the following problems.
FIG. 2 illustrates a handover between RSs in a conventional multi-hop relay system.
Referring to FIG. 2, when the BS simply forwards data, destined for the MS2, to an RS1 covering MS2 by using a frame A, the RS1 must buffer all of data for MSs serviced by RS 1. At this point, when MS2 is handed over from the serving RS1 to a target RS2, the previous data buffered by the serving RS1 becomes useless and thus the BS must retransmit the data, which was transmitted to the serving RS1, to the target RS2 by using a frame A. This leads to a waste of a BS-RS Transmission (TX) frame (i.e., a frame A), causing the degradation of the overall system performance. When a resource waste occurs as described above, the resource allocation for an MS1 communicating directly with the BS is delayed to delay the communication service for MS1. These problems become more serious as the number of the radio hops increases above 2.